During the operation of a turbine power plant, there are various conditions which may occur necessitating an immediate shutting down or "tripping" of the turbine. For example, a loss of electrical load may create a dangerous overspeed condition; low bearing oil pressure may cause excessive wearing and serious malfunction of the turbine bearings; excessive wearing of the thrust bearing results in axial misalignment of the rotating blades resulting in serious internal turbine damage; insufficient condenser vacuum may cause overheating at the last row of turbine blading; or other contingencies may occur where it is necessary to shut down or "trip" the turbine rapidly to prevent an unsafe operating condition or damage to the turbine power plant.
A failure or delay in shutting off the steam to the turbine in the event of any of the above contingencies may cause extensive damage to various portions of the plant, necessitating expensive repairs and prolonged shutdown. Thus, it is necessary that such a system react quickly to specific contingencies.
In a typical steam turbine power plant, oil is and low at high pressure to a plurality of hydraulically operated valves for controlling steam flow. These valves are designed to open on an increase in oil pressure, and to close on a decrease in oil pressure. Governor valves control steam flow to the high pressure turbine and interceptor valves control the flow of steam to the intermediate and low pressure turbine stages. Throttle valves, which control the flow of steam to the steam chest upstream of the governor valves and reheat stop valves, which control the flow of steam from the reheater section of the steam generator to the intermediate low pressure turbine stages upstream of the interceptor valves, are provided primarily for protective control of the turbine. The throttle valves are also used for turbine startup. Thus, when tripping the turbine, the throttle valves, the governor valves, the reheat stop valves, and the interceptor valves are rapidly closed. This is accomplished by releasing the oil pressure to all of the valves simultaneously in response to the detection of any one of several operational contingencies or by remote means under the control of the operator.
Turbine tripping systems presently in use utilize a mechanical hydraulic automatic stopping mechanism, which is referred to as an "autostop", to maintain under pressure, the valve control oil for the steam inlet valves an emergency trip valve, as well as the throttle valves and the reheat stop valves, are under the control of a hydraulic system, referred to as an autostop control oil system. The maintenance of pressure in this autostop oil system permits the throttle and reheat stop valves to be opened and the pressure in the control oil system to be maintained by the emergency trip valve. An overspeed trip valve maintains the proper pressure in the autostop oil system to keep the emergency trip valve closed, and all steam inlet valves operable. The overspeed trip valve is operated to release the autostop oil pressure wither by a centrifugally operated overspeed tripping device on the trubine shaft or the operation of the mechanical autostop trip lever by an operator. The centrifugally operated overspeed tripping device moves a lever when the turbine reaches a predetermined overspeed conditions to cause the overspeed trip valve to release the autostop oil pressure, thus completely shutting down the turbine.
The conventional mechanical autostop assembly, typically consists of a machined block, on which is mounted a lever arrangement which, when moved a predetermined distance, hydraulically causes the overspeed trip valve to release the autostop oil pressure. Connected to move the lever in the autostop assembly are a plurality of linkages which are connected to respective protective devices in the autostop machined block. For example, a low bearing oil pressure trip in the form of a spring-loaded diaphragm is exposed to bearing oil pressure, which release high pressure oil to operate the overspeed trip valve when bearing oil pressure is below a predetermined value. A low vacuum trip in the form of a pressure responsive bellows is exposed to exhaust vacuum, and operates the overspeed trip valve when the exhaust vacuum drops below a preset value. A thrust bearing trip in the form of a spring-loaded diaphragm operates the overspeed trip valve when pressure builds up in response to a certain position of the thrust collar. Finally, the autostop assembly includes a solenoid, which when energized by the operator, moves the lever on the autostop assembly to operate the overspeed trip valve.
Also, in conventional tripping systems, a separate overspeed trip mechanism, which consists of an eccentric weight mounted in the end of the turbine shaft, is balanced in position by a spring until the turbine speed exceeds a predetermined amount. The centrifugal force then overcomes the spring and the weight flies out, striking a trigger which trips the overspeed trip valve, releasing the autostop oil pressure and shutting down the turbine.
The conventional trip systems, presently in use, are effective in tripping the turbine, either manually by the operator, or in response to the various operating contingencies, including those previously mentioned. However, such systems are relatively slow in their operation because of the mechanical linkage.
In turbine power plants where the controls are automated or controlled from a central office, it is desirable to maintain the reliability and rapid response of a hydraulic system and to eliminate the relatively slow operation, difficulty of adjustment, and limited range of response, of the mechanical autostop assembly with its accompanying linkage.